An often used commercial system for the separation of a mixture comprising oxygen, nitrogen and argon, e.g. air, is cryogenic rectification. The separation is driven by elevated feed pressure which is generally attained by compressing feed in a compressor prior to introduction into a column system. The separation is carried out by passing liquid and vapor in countercurrent contact through the column or columns on vapor liquid contacting elements whereby more volatile component(s) are passed from the liquid to the vapor, and less volatile component(s) are passed from the vapor to the liquid. As the vapor progresses up a column it becomes progressively richer in the more volatile components and as the liquid progresses down a column it becomes progressively richer in the less volatile components. Generally the cryogenic separation is carried out in a main column system comprising at least one column wherein the feed is separated into nitrogen-rich and oxygen-rich components, and in an auxiliary argon column wherein feed from the main column system is separated into argon-richer and oxygen-richer components.
The power to operate the feed compressor and thus drive the separation is the major operating cost of the separation. Pressure drop within the system burdens the feed compression causing increased feed pressure requirements. It is desirable to operate the cryogenic rectification with as low a pressure drop as possible thus reducing feed compression requirements. Furthermore, the lower the pressure level within the columns the greater is the relative volatility between the components. The greater is the relative volatility between the components within a column, the easier is the separation, which in turn increases the recovery of argon, oxygen and nitrogen products.
In the operation of the argon column, a vapor stream having a relatively high argon concentration is taken from the main column system and passed into and up the argon column while becoming progressively richer in argon. A crude argon product is recovered from the top of the argon column. Vapor flows up the argon column due to a pressure gradient between the argon column feed and the crude argon product. The pressure of the argon column feed is determined by the main column conditions at the vapor takeoff point. Operation of the argon column at a lower pressure is subject to two constraints on how low the pressure can be irrespective of how the lower pressure is achieved. One constraint is that subatmospheric pressure at the top of the argon column should be avoided in order to avoid air leaks into the system. The other constraint involves the temperature difference for the top condenser of the argon column. A low pressure at the top of the argon column also results in a low temperature and thus the temperature difference between the condensing argon and the boiling kettle liquid in the argon condenser is reduced. A minimum temperature difference of about 0.7.degree. K. is necessary for effective operation of the condenser. When the argon column pressure is reduced through the use of a valve, a particular disadvantage arises in that the liquid at the bottom of the argon column which must be returned to the main column is now at a lower pressure than the pressure in the main column at the return point. Thus repressurization of the liquid, for example by pumping or by raising the elevation of the argon column, is necessary. This repressurization is costly and creates a system inefficiency.
Accordingly it is an object of this invention to provide a cryogenic separation method for separating a feed comprising oxygen, nitrogen and argon wherein an argon column is operated at a lower average pressure without the need for repressurizing liquid passing from the argon column to the main column.
It is another object of this invention to provide a cryogenic separation apparatus comprising a main column system and an auxiliary argon column, which can operate at a lower average pressure without the need to increase the pressure of liquid passed from the argon column to the main column.